1. Field of the Invention
The invention relates to range finding particularly with respect to passive optical range finding.
2. Description of the Prior Art
Active range finding apparatus is known in the prior art utilizing radar or laser technology. Although such apparatus measures range very accurately, the military use of such range finding devices exposes the user's position to an adversary by means of detection devices. It is a desideratum of the prior art to rapidly and accurately measure range to a target utilizing passive apparatus for such purposes as position calculation and fire control. It is also desirable that the passive apparatus occupy a reasonably small space for use in strategic applications such as in the periscope technology.
Range finding in the periscope technology traditionally utilizes a stadimeter or stadimetric reticles. In the stadimetric method, a target is identified and range is calculated from the known height of a target feature and the angle subtended thereby (angular subtense). The calculation utilizes the a priori knowledge of such target dimensional characteristics.
Another passive range finding technique involves the traditional parallax range finder. This traditional instrument, developed as a naval fire control range finder, views a target from two optical apertures. When the lines of sight of the two apertures are parallel, two images are observed. By varying the angle of one viewing aperture, coincidence of the images is effected and the angular variation of the one viewing aperture is a measure of target range. Thus, the parallax range finder utilizes the principle of triangulation or determining range by solving the triangle produced by the two view-points and the target.
On, for example, a naval battle cruiser, a "baseline" or distance between apertures of approximately 1 meter is required for accurate range measurements to the horizon (approximately 14 kilometers). This procedure has significant disadvantages since it involves an operator with the human limitation of visual acuity and also utilizes an inaccurate mechanically varied prism angle. Because of the limited human visual acuity and the inaccuracies of the mechanical device, the large baseline is required for reasonable range and accuracy. The relatively low resolution with which the operator could effect image coincidence made the large baseline necessary.
Both the parallax and stadimetric range finders require very well trained observers. The stadimeter, in particular, requires special skills and a reference library. Both methods require a significant amount of time to determine range, during which time the observer or the periscope is exposed.